Recall that an "RF pulse" is merely a magnetic field (B1) of short duration rotating at the Larmor frequency in the transverse plane. Depending on its phase, B1 may be considered to be "directed" along the x-axis or y-axis (or anywhere else in between). The above illustration shows the different refocusing directions produced by 180°-pulses along different axes.
By adjusting the phase of transmission, the 180° pulse may be applied along the x-axis, y-axis, or any other direction. On the left side of the diagram is a 180y° pulse, that is, one that rotates the spins around the y-axis. The echo forms in the +y-direction. On the right is a 180x° pulse that causes the echo to form along the −y-direction.
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How is a spin echo generated?